OK I did some CW measurements. I used a standard ruler. All of these are Hasbro, and are for the length of the top of the CW(No heights or side lengths) I did maximum and minimum lengths for most of them, but some of the minimum lengths were hard to determine(e.g. Pisces has many possible minimum lengths). So:
Rex: Max Length: 3.5 cm - Min Length: 3 cm
Wolf: Max Length: In between 3.6 and 3.7 cm - Min Length: 3.4 cm
Pisces: Max Length: 3.5 cm - Min Length: 3.4 cm
Cygnus(No max or minimum length because it is a smooth circle): Length: 3.6 cm
Byxis(Crack is growing through inside where it rest on MW, but is not through it, so this may or may not count): Max Length: 3.6 cm - Min Length: 3 cm
Pegasus II: Max Length: I got 3.7 cm - Min Length: 3.3 cm
I hope these are accurate. If they are, then Pegasus II was the widest in diameter of these.
EDIT: Just realized some of the minimum lengths were not really needed, but I will just keep them here for reference.